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Septic arthritis
A single hot, swollen joint is infection until proven otherwise — aspirate the joint and take blood cultures before antibiotics, and drain it; Staphylococcus aureus is the common organism, and Gonococcus is suspected in a sexually active young person.
Why it matters in the exam
The question presents one joint (usually the knee) that is hot, swollen and painful on passive movement, with or without fever, often in an older person, a patient with RA or diabetes, or someone who injects drugs. The correct step is immediate aspiration for cell count, Gram stain, culture and crystals — not "a trial of NSAIDs". Turbid fluid with more than 50,000 white cells per mm³, mostly neutrophils, strongly suggests infection, but a lower count does not exclude it. Crystals in the fluid do not exclude a coexisting infection.
With Gonococcus the picture differs: a sexually active young person with migratory joint pain, tenosynovitis of the hand and pustules on the skin, or a single inflamed joint; the joint culture is often negative, so PCR is sent from the cervix, urethra and throat. Empirical treatment: with a risk of MRSA — vancomycin; otherwise cefazolin or flucloxacillin according to local practice; for Gonococcus — ceftriaxone. Repeated needle or arthroscopic drainage is part of treatment, and a prosthetic joint calls for an orthopaedic referral. Treatment usually lasts 4–6 weeks. While results are awaited, no steroid is injected into the joint and pain relief alone is not enough, because cartilage destruction begins within a few days and an undrained joint may be damaged for good.
Key points
- One hot swollen joint = infection until proven otherwise.
- Aspirate and culture before antibiotics; WBC above 50,000 is highly suspicious.
- S. aureus is most common; vancomycin if MRSA risk.
- Gonococcus: young person, tenosynovitis, pustules; ceftriaxone.
- Repeated drainage is part of treatment; duration 4–6 weeks.
Summary table
| Situation | Clue | Test | Treatment |
|---|---|---|---|
| S. aureus (most common) | Older person, RA, diabetes, injecting drugs | Turbid fluid, WBC above 50,000, Gram-positive | Vancomycin if MRSA risk, otherwise cefazolin; drainage |
| Gonococcus | Sexually active young person, migratory joints, tenosynovitis, pustules | PCR from cervix, urethra and throat | Ceftriaxone |
| Crystals | Gout or CPPD | Crystals under polarised light | Do not exclude coexisting infection |
| Prosthetic joint | Pain and swelling of an artificial joint | Aspiration by an orthopaedic surgeon | Surgery + prolonged antibiotics |
| Duration | After drainage and response | Clinical follow-up and CRP | Usually 4–6 weeks |
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Sources
- BSR/BHPR/BOA Guideline for management of the hot swollen joint in adults, 2006 — Assessment, joint aspiration and empirical antibiotics
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Infectious Arthritis
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.